The present invention is related to a differential ring oscillator and a calibration method thereof, and more particularly to a differential ring oscillator and a related method capable of calibrating the differential ring oscillator to compensate for semiconductor process variations.
In a phase-locked loop, an oscillator is used to generate an oscillating signal according to a tuning signal when the phase-locked loop is operated under a frequency-locked mode. Ring oscillators are often utilized in the phase-locked loop due to their simple structure and easy implementation. Moreover, the ring oscillator occupies a small area and has no magnetic coupling problem when combined with other systems since no inductor is used therein. Another advantage of the ring oscillator is that the even number ring oscillator stages generate quadrature output phases, saving the need for a divide by 2. The frequency of the oscillating signal generated by the ring oscillator may vary greatly, however, due to semiconductor process variations i.e. the corners of the semiconductor manufacturing process. Therefore, there is a need for developing a calibrating scheme capable of calibrating the ring oscillator affected by process variations in order to generate an oscillating signal having a predetermined target frequency.